As technology development and demand for mobile devices are increasing and the spread of electric vehicles is expanding, demand for secondary batteries as energy sources is rapidly increasing. Among them, demand for lithium secondary batteries having high capacity and energy density is especially high.
Generally, a lithium secondary battery is fabricated by manufacturing an electrode assembly composed of a negative electrode, a positive electrode, and a separator, inserting the electrode assembly into a battery case, and injecting an electrolyte into the electrode assembly. The lithium secondary battery thus produced is required to be activated by a predetermined charge and discharge to function as a battery. Such a process is referred to as a formation process or an activation process. The secondary battery is also shipped after an aging process and a defective product sorting process. The aging process is an aging process in which the electrolyte is allowed to enter the empty space of the electrode to allow time for forming a stable electrolyte channel.
The secondary battery is manufactured so that a positive electrode and a negative electrode are prevented from being contacted by a porous insulating film (separator) to prevent a short circuit. However, insulation may not be properly maintained due to various reasons during the manufacturing process of the battery. As a result, an internal short circuit of the battery can occur. Lithium-ion batteries can be ignited or exploded when a positive electrode and a negative electrode are short-circuited. Even when they are slightly short-circuited, ions move and current flows. This condition is often referred to as a soft short or a micro short.
Soft-shorts generate precipitates, thereby causing low-voltage defects. Soft-short cells tend to take a relatively long time to be expressed as compared to hard-short cells, and their expression time vary considerably depending on the short-circuit state or degree.
In the conventional defect selection process, the difference in the voltage drop between good products and defective products has been utilized. And through the value of the voltage drop measured at this time, the change by the low voltage expression steps and the effects according thereto would have been presented. However, this method cannot quantitatively analyze the actual low-voltage expression level, so it is impossible to quantitatively classify the degree of failure of defective cells for efficient process improvement, and the level of low-voltage expression of defective cells cannot be represented as a single measure.
Therefore, it would be easy to identify defective battery cells according to the degree of the voltage drop if the low voltage level is quantitatively indicated and used as the leakage current, which is a factor related to the precipitate that directly induces the low voltage defect, and quantitative feedback for improvement of the battery cell manufacturing process would be possible.